Technical Field
The present disclosure generally relates to a method for performing a closed-loop bit adaptive precoding matrix indicator feedback mechanism and an apparatus using the same.
Related Art
The Multiple-Input multiple-output (MIMO) technique has been proved to be a successful approach to enhance capacity of single input single output (SISO) systems. For an NR×NT MIMO system, the system capacity asymptotically increases linearly with a slope that is equal to the minimum of NR and NT, where NR is the number of receive antennas and NT is the number of transmit antennas. In addition to the capacity gain, the MIMO technique also could provide a maximum diversity gain that is equal to NT×NR. This MIMO technique is now entering the 4th generation wireless cellular and wireless LAN products.
These MIMO gains are realizable when the corresponding receiver of a transmitter also knows the MIMO wireless channel. If the transmitter knows the wireless MIMO channel, system performance could be further improved by using signal processing techniques. One of these techniques is precoding. Precoding is to transform transmitted data before sending the data through antennas. Precoding could be classified as linear and non-linear precoding. Non-linear precoding techniques include dirty paper coding (DPC), Tomlinson-Harashima precoding (THP), etc. Linear precoding transforms data by multiplying the data with a precoding matrix to match the channel eigenmodes. Linear precoding is simple to implement in a system in which the system performance is easier to analyze than a system with non-linear precoding. For these reasons, linear precoding has been adopted in communication standards such as 3GPP long term evolution (LTE) and LTE-advanced (LTE-A). Thus linear precoding is expected to dominate future implementations of telecommunications networks. Linear precoding could also be applied to enhance capacity, which is called interference alignment.
There are two kinds of design approaches to implement linear precoding. One such approach is a codebook-based precoding; the other is a non-codebook-based precoding. Basically, the non-codebook-based precoding has better performance than the codebook-based precoding since the non-codebook-based precoding requires instantaneous channel state information (CSI) to design the best precoder for the moment. For a frequency-division duplexing (FDD) system, uplink (UL) signals need extra bandwidths to feed back CSI from a receiver to a transmitter for performing downlink (DL) precoding since the downlink and uplink channels are allocated in different frequency bands. In this way, the feedback overhead of CSI is high if full channel information is needed in order to feed back CSI from the receiver to the transmitter.
Codebook-based precoding could reduce the signal feedback overhead. There is a trade-off between system performance and signal feedback overhead. The optimum codebook-based approach basically follows the guideline of Grassmannian packing. The codebook design is irrelevant to instantaneous wireless channels. The codebook is designed by maximizing the minimum distance of any two codewords (precoder) in a codebook. For fast codebook design, DFT-based codebook design could be used. Owing to this kind of CSI-independent design, we may not need to feed back CSI. Since the codebook is designed regardless of instantaneous channels, the codebook could be designed off-line and stored in both the transmitters and receivers. In this way, a receiver only has to feed back the precoding matrix indicator (PMI) in a codebook to indicate which precoder the transmitter should use. Since the performance of codebook-based precoding is limited by the pre-designed codebook, some works have been focusing on adaptive codebook designs in order to further improve the system performance. An adaptive design approach could improve the system performance by requiring the codebook to make adaptive changes based on channel statistics such as channel spatial correlation and channel temporal correlation. Extra information of channel statistics must be fed back to a transmitter to update the current codebook. Thus, extra computing power could also be required to perform codebook updates. The codebook could be changed according to antenna settings such as uncorrelated or diversity setting, cross-polarized setting, and uniform linear array setting. These above-mentioned methods are different from the approach in the Standard LTE-A, which uses a fixed codebook under a certain configuration.
Coordinated Multipoint Transmission (CoMP) is a new technology to increase data transmission rate in LTE. By coordinating and combining signals from multiple base stations (e.g. transmission points or eNBs in LTE), CoMP could enable mobile users to enjoy consistent performance and quality regardless whether the mobile users are close to the center of a cell or are at a cell boundary. Since the number of eNBs is greater in CoMP scenarios, the number of feedback bits for PMI will become larger. In addition, when feedback timing of each transmission point collides, the feedback format designed in the current specification may not support such larger number of feedback bits for many transmission points at the same time. Thus, designing a flexible and efficient PMI feedback mechanism has become a major issue.